1. Field of Invention
The present invention relates to ranging methods in wireless communication. In particular, the present invention relates to ranging methods used in conjunction with uplink (UL) multi-user timing synchronization and power control applications in orthogonal frequency division multiple access (OFDMA) systems.
2. Discussion of the Related Art
In an OFDMA mobile communication system, various ranging methods for UL synchronization or power control between a mobile station (MS)1 and a base station (BS) have been developed to improve robustness (e.g., prevent the ranging code collision) and reduce network latency (e.g., reduce access delay). 1 The MS is also referred to as a subscriber station (SS) or a user element (UE).
An initial ranging and periodic ranging method is disclosed in U.S. Patent Application Publication, no. 2007/0058524 (“Modlin”), which is entitled “Orthogonal Frequency Division Multiplexing Access (OFDMA) ranging,” naming as inventors C. S. Modlin and T. Muharemovic and which was published on Mar. 15, 2007. In Modlin's method, the received signal consists of OFDMA symbols. Under Modlin, a fast Fourier transform (FFT) is firstly performed on the received signal, from which the ranging codes of the received signal are extracted and compared with a ranging code hypothesis. After the matching ranging codes are identified, a power estimate for each ranging code is compared to a power threshold to confirm correct receipt of the ranging code. Finally, the timing offset and the power of the received signal are reported. Modlin discloses both a ranging code hypothesis and phase drift detections based on the matching extracted ranging codes. The ranging code hypothesis is used to estimate delay and power. Although Modlin's method may be extended to support multiple reception antennas, the complexity of such an implementation is high. Furthermore, the matching process requires long processing time at the BS and thus, increases both the latency and the access delay of the overall network. More importantly, the ranging code detection scheme does not adequately take into consideration channel frequency-selectivity, so that orthogonality among ranging codes is poor, resulting in significant multiuser interferences. Furthermore, the timing offset and power estimation requirements fail to consider other ranging codes, thus causing interferences and may degrade performance significantly.
An apparatus and a method for processing a ranging channel at the BS are disclosed in U.S. Patent Application Publication, no. 2005/0135230 (“Yu”), which is entitled “Apparatus and Method for Processing Ranging Channel in Orthogonal Frequency Division Multiple Access System,” naming as inventors C. W. Yu, K. Y. Sohn, Y. O. Park, and S. K. Hwang, and which was published on Jun. 23, 2005. Yu measures the propagation delay and the power of each MS in the UL of an OFDMA system. In particular, Yu discloses a BS's receiver which processes a ranging channel using a ranging channel signal extractor and a ranging channel processor. The ranging channel signal extractor selects the subcarriers allocated to the ranging channel. The ranging channel processor then correlates the subcarriers in time to estimate the propagation delays of the ranging channel and integrates the subcarriers to measure the magnitude or power of the signal of the ranging channel. Yu converts the received ranging complex signals to polar coordinates for delay estimation and converting the polar coordinates back to complex coordinates for magnitude or power estimation in the integrator. Yu's technique reduces receiver complexity; however, its technique is limited to single-antenna systems.
An apparatus and a method at a BS for receiving a ranging signal in an OFDMA communication system are disclosed in U.S. Patent Application Publication, no. 2006/0098749 (“Sung”), which is entitled “Apparatus and Method for Detecting Ranging Signal in an Orthogonal Frequency Division Multiple Access Mobile Communication System,” naming as inventors S. H. Sung, I. S. Hwang, S. Y. Yoon, C. R. Chang, and J. H. Cho, and which was published on May 11, 2006. Sung discloses, in an OFDMA mobile communication system, a method of reducing a computation requirement for ranging signal detection and for improving ranging detection performance even at a low carrier-to-interference-plus-noise-ratio (CINR). Sung's technique is limited to single-antenna systems.
An UL timing synchronization method between a SS and a BS in an OFDMA communication system is disclosed in U.S. Pat. No. 7,359,366 (“Lee”), which is entitled “Uplink Ranging System and Method in OFDMA System,” naming as inventors Y. H. Lee, J. W. Kim, Y. O. Park, S. K. Hwang, and which was published on Jun. 30, 2005. Lee discloses both a new ranging system and a new ranging method. In particular, Lee discloses ranging in the BS without an additional FFT process and without mitigating restrictions on both the ranging speed and the location of the ranging system. Lee also discloses sequentially executing functions in various blocks of the ranging system. Lee also discloses ranging at a relatively high processing speed without carrying out an FFT to detect a complex exponential component of a timing error.
While the ranging methods of Yu, Sung and Lee reduce implementation complexity at the BS (as compared, for example, to conventional techniques2), all of these methods are limited to single-antenna systems and are inapplicable for multiple antennas systems. Hence, for next-generation broadband wireless access systems that deploy multiple-input-multiple-output (MIMO) technology (e.g., those systems under the IEEE 802.16m standard), the methods of Yu, Sung and Lee may become impractical. Yu, Sung and Lee also do not take into account the sensitivity of their ranging signals to other data users and ranging users. Also, these schemes do not consider effect of channel frequency selectivity and do not efficiently exploit various forms of diversity. 2 See, e.g., “IEEE Standard for Local and Metropolitan Area Networks Part 16: Air Interface for Fixed Broadband Wireless Access Systems,” IEEE Std 802.16-2004, 1 Oct. 2004.
Turning to ranging signal designs, a ranging method for minimizing access delay and for preventing ranging code collisions is disclosed in U.S. Patent Application Publication, no. 2005/0058058 (“Cho”), which is entitled “Ranging Method in a Mobile Communication System Using Orthogonal Frequency Division Multiple Access,” naming as inventors M. H. Cho, B. G Song, S. H. Sung, and K. S. Eom, and which was published on Mar. 17, 2005. To reduce access delay and to prevent ranging code collisions, Cho discloses performing, according to a state of a SS, an adaptive bandwidth request ranging based on a scheduled access technique or a random access technique. Specifically, Cho's method allows an SS in an active state to preferentially perform a scheduled access-based bandwidth request ranging. Cho's method avoids an access delay caused by a ranging code collision and improves data transmission efficiency. However, Cho's method is applicable only for a bandwidth request ranging that is performed after a successful initial ranging.
A system and method for ranging for a fast handover in an OFDMA communications system are disclosed in U.S. Pat. No. 7,307,973 (“Song”), which is entitled “System and Method for Ranging for a Fast Handover in a Mobile Communication System,” naming as inventors B. G. Song, K. S. Eom, M. H. Cho, H. J. Ju, and which was published on Jun. 2, 2005. Song discloses a ranging method that reduces delay time for a SS subject to a handover. Song's method utilizes specific ranging codes and ranging slots in an OFDMA system to perform a handover ranging which avoids ranging code collisions while minimizing access delay time. In particular, Song allocates a handover ranging code and a handover ranging slot for the handover ranging. Under Song's method, when an SS handovers from a serving BS to a target BS, the handover ranging may be performed without an initial ranging. By allocating ranging codes for handover ranging (i.e., “handover ranging codes”), ranging code collisions between handover ranging and non-handover ranging operations are avoided, thereby achieving a fast handover. However, Song's handover ranging scheme is only applicable for an SS that is in the process of a handover. Furthermore, by dedicating some codes to handover ranging, less ranging codes are available for conventional ranging operations (e.g., initial ranging, periodic ranging and bandwidth request ranging). As a result, both the probability of code collision during non-handover ranging operations and the times incurred for retransmissions due to collisions are increased.
A ranging method for an OFDMA communication system that divides the ranging procedure between a transmission side (e.g., BS) and reception sides (e.g., UEs) into three categories is disclosed U.S. Pat. No. 7,310,303 (“Koo”), which is entitled “Ranging Method for Mobile Communication System Based on Orthogonal Frequency Division Multiple Access Scheme,” naming as inventors C. H. Koo, D. S. Park, and P. Y. Joo, and which was published on Oct. 23, 2005. Koo discloses a ranging code allocation method and a method for reducing a UE's access delay time, by assigning an independent backoff value to each ranging code. In particular, Koo classifies ranging codes for initial ranging, bandwidth request ranging, and periodic ranging. The BS informs a UE of the range of ranging codes that are currently available for the UE, assigns different ranging codes and their backoff values to the ranging processes according to ranging objectives, and informs a UE of such allocation result. The BS dynamically assigns the number of ranging codes and their backoff values to the ranging processes according to the BS's cell status and a ranging objective. Koo's method results in a minimal UL access delay time and a minimal number of UL access collisions. Koo's method, however, requires new settings in the UL_MAP and the UL Channel Descriptor (UCD) messages which the BS sends to UEs. Consequently, both the complexity of the system and the overall network latency are increased. Furthermore, classifying ranging codes into several categories also reduces the number of ranging codes available for selection, so that the number of UEs that can be supported in each cell is reduced.